1. Field of the Invention
The present invention relates to a spread spectrum communication apparatus. More specifically, the present invention relates to a spread spectrum communication apparatus for transmission/reception using direct sequence spread spectrum communication with a data format having known data portion in a preamble portion.
2. Description of the Background Art
Communication using a narrow band modulation system has been conventionally used in the field of data communication. Such a system is advantageous in that demodulation at the receiver can be carried out by a relatively small circuitry. However, it is not suitable for an environment where there are much reflection and multipath fading, such as in a room. By contrast, in a spread spectrum communication system, spectrum of data is spread by a spread code and the data is transmitted in a wide band. Therefore, the aforementioned disadvantage can be eliminated.
FIG. 24 is a block diagram showing a general structure of such a spread spectrum communication system. FIG. 24 is a block diagram showing a demodulation system, which includes a PDI (post detection integrator).
Referring to FIG. 24, a signal input from an input terminal is input to correlating portion 1. Correlating portion 1 provides correlation of the input signal, and the correlated output is applied to a differentiating portion 2 delayed by 1 symbol of data by delay portion 5, and it is differentially demodulated. An output from differentiating portion 2 is input to a PDI portion 3. Meanwhile, in a correlation timing detecting portion 6, a correlation timing is determined, and PDI operation is performed at the correlation timing and data is determined.
Here, PDI refers to a method of integrating demodulated signals for a period of time spread of multiplexed communication. RAKE method is also known as a similar method in which multipath signals are demodulated by respective addition using a transversal filter and a weighting circuit.
FIG. 25 shows correlated output waveforms in the system shown in FIG. 24. In a general radio wave propagating environment, there are a number of delay waves which are considered to be subjected to Rayleigh fading. In this case, if there is not a delay, the correlated waveform has peaks only when correlation is established as shown in FIG. 25(a). However, if there are a number of delay waves, the delay waves have peaks of correlated waveforms respectively, and the resulting waveform, which is a linear sum of these, is as shown in (b) of FIG. 25. Such spread caused by delay indicates the state of delay. Therefore, this is called a delay profile.
PDI is capable of demodulation from these number of delay waves and improving performance. For example, when there is not a delay wave, demodulation at the timing A shown in FIG. 25(a) is most efficient. However, when there are a number of delay waves, the spread of delay wave reaches the timing t1 of (b) of FIG. 25. Therefore, up to that time, the signal component is included. Therefore, integration of results of demodulation improves performance. However, if in this case, integration is continued up to timing t2, there is not a signal component later than the time t1, and therefore integration merely increases noise, degrading performance. If integration is performed up to the time t3, the time period for integration is too short to ensure sufficient performance. Practically, highest performance can be obtained by determining a delay wave period having a certain level or higher. The trade off is determined based on the ratio of signal component and noise component increased when the time period for integration is increased.
For example, when the time period for integration is doubled, the noise component would also be doubled. By contrast, the signal component would be the result of integration of components obtained from the delay profile of correlated waveform shown in FIG. 25(b). Optimal time period for integration can be determined based on the ratio therebetween. However, in general communication environment, the delay profile changes with time. Therefore, conventionally, the time period for integration is determined to be optimal based on numerous past rate data, and it is incorporated in hardware when implementing the system.
FIG. 26 shows effects provided by the conventional PDI. Referring to FIG. 26, when PDI or RAKE method is employed, performance can be improved than when not (under Rayleigh fading condition). Here, when path diversity such as PDI or RAKE is used, the performance depends on how much delay wave the communication environment employed bears, and on how much the delay waves are collected (integration or addition). Generally, as described in the foregoing, the period of integration is determined based on the past experimental data. However, the environment used changes with time generally, and hence the spread of delay wave changes from time to time. Therefore, it is impossible to always maintain an optimal period of integration. Therefore, compared with theoretical optimal value, the performance in an actual system is inferior.
FIG. 27 shows correlated output waveforms in the conventional system. Conventionally, depending on the environment used, there is much amount of delay, and sometimes data component of previous signal may overlap the succeeding signal, as shown in (b) of FIG. 27, resulting in degraded performance. In view of the foregoing, when designing a communication system, it is necessary to determine a data symbol speed so as not to overlap the adjacent signals and to ensure sufficiently large time interval between symbols with respect to the delay amount. These are also determined based on the past experimental data, as above. However, as already mentioned, the amount of delay is not constant. Therefore, sometimes a signal of the previous symbol may overlap the following signal, causing degraded performance as shown in (b) of FIG. 27.
FIG. 28 is a schematic block diagram showing a transmitter of a spread spectrum communication system in which a spread code is delayed and multiplexed in the method of high speed transmission using spread spectrum, proposed by the inventor of the present application in Japanese Patent Application No. 7-206159. Referring to FIG. 28, the transmitter of the spread spectrum communication system includes a data generating portion 11, an S/P converting portion 12, multipliers 13, 14, 15 and 16, modulators 17, 18, 19 and 20, a PN generator 21, a local signal generator 22, delay elements 23, 24, 25 and 26, a multiplexer 27, a frequency converting portion 28, a power amplifying portion 29 and a transmission antenna 30.
Data generating portion 11 generates differentially coded data, and each data is converted to a plurality of parallel data by S/P converting portion 12. PN generator 21 generates a spread code. Multipliers 13 to 16 multiply the plurality of parallel signals from S/P converting portion 12 by the spread code from PN generator 21, and generates and applies spread signals to modulators 17 to 20.
Modulators 17 to 20 modulate the spread signals by using a local frequency signal from local signal generator 22, and provides intermediate frequency signals. Delay elements 23 to 26 delay the intermediate frequency signals and apply these signals to multiplexer 27. Multiplexer 27 multiplexes the delayed signals, converts the frequencies by frequency converting portion 28, and the frequency converted signal is amplified by power amplifying portion 29 and output as a transmission signal from transmission antenna 30.
FIG. 29 shows correlated outputs when the signal transmitted from the transmitter shown in FIG. 29 is received and passed through a correlator. As compared with the example shown in FIG. 25, in the example shown in FIG. 29, the time between each of the data is shorter, and the multiplexed delay has arbitrary value dependent on the amount of delay. Therefore, the possibility of overlap of multipath delay waves is higher, dependent on the environment of propagation. In order to avoid such a problem, the number of multiplexing/amount of delay must be varied. The reference for determining these values may be initialized uniquely in accordance with the place of installation and varied in accordance with the error rate. However, this method is not very precise, still resulting in some degradation in performance. Further, if margin is too large and the number of multiplexing is too small, data rate will be decreased and system through put is degraded.